1. Field of the Invention
The present invention relates to a method of manufacturing an extruded nozzle for casting molten metal therethrough and a composition for such an extruded nozzle.
2. Prior Art
Various casting nozzles or casting nozzle bricks such as long nozzles and immersion nozzles have recently found widespread use in the fields of continuous casting for steel and production of clarifying steel to prevent molten steel from being oxidized during casting operation. Known nozzles are composed of a large amount of graphite to render themselves resistant to spalling and corrosion, and are formed by rubber presses. While rubber pressing can exert uniform pressure on nozzles being formed, graphite layers tend to be displaced, and resulting nozzles are subjected to poor dimensional accuracy. The rate of production of such nozzles is low because a finishing process such as grinding and cutting is required to remove dimensional irregularities, with the result that the nozzles are relatively costly to fabricate.
Extrusion processes have also been employed for producing graphitic casting nozzles or nozzle bricks. One prior example is disclosed in Japanese Laid-Open Patent Publication No. 48-28329, published Apr. 14, 1973, in which a solid cylindrical body is first extruded, and then a through bore for passage of molten metal therethrough is defined after the cylindrical body has been baked. The disclosed process requires complex manufacturing steps and hence produces casting nozzle bricks at a retarded rate.
A graphitic casting nozzle brick is known which has an axial bore with one end closed and which includes lateral holes defined through the nozzle across the bore adjacent to the closed end. A conventional method of fabricating such a casting nozzle brick has been to form a cylindrical body having a through bore and then to close off one end of the bore with an end cover. Such a prior method is disadvantageous, however, in that the nozzle is likely to crack at a junction where the end cover is joined to the cylindrical body at the time of baking or use.
Another known type of casting nozzle brick comprises a cylindrical body having an axial through bore, and is formed by an auger or extrusion machine having a central core attached by a beam to an extrusion die for defining the through bore. As a material to be extruded is forced to flow through the extrusion die, the material is cut and separated by the beam and then reunited past the beam into an extruded mass. No problem would arise out of the extrusion process when producing nozzles which have no or little graphite content. However, those nozzles which contain an increased amount of graphite and are fabricated by such an extrusion process are apt to crack along the surface of rejoining after the nozzles have been baked, especially when subjected to a thermal shock while in use.